1. Field of the Invention
The present invention is directed to systems and methods for fabricating integrated circuit band-pass filters, and particularly a process for manufacturing integrated circuit (IC) filters using micro electro-mechanical system (MEMS) technology that involves the encapsulation of a single-crystal MEMS band-pass filter in a vacuum environment, at or below the substrate surface using standard processing, compatible with other standard integrated circuit devices, and, in a manner that eliminates the need for precision lithography.
2. Discussion of the Prior Art
Micro Electro-Mechanical Systems (MEMS) technology is currently implemented for the fabrication of narrow bandpass filters (high-Q filters) for various UHF and IF communication circuits. These filters use the natural vibrational frequency of micro-resonators to transmit signals at very precise frequencies while attenuating signals and noise at other frequencies. FIG. 1 illustrates a conventional MEMS bandpass filter device 10 which comprises a semi-conductive resonator structure 11, e.g., made of polycrystalline or amorphous material, suspended over a planar conductive input structure 12, which is extended to a contact 13. An alternating electrical signal on the input 12 will cause an image charge to form on the resonator 11, attracting it and deflecting it downwards. If the alternating signal frequency is similar to the natural mechanical vibrational frequency of the resonator, the resonator may vibrate, enhancing the image charge and increasing the transmitted AC signal. The meshing of the electrical and mechanical vibrations selectively isolates and transmits desired frequencies for further signal amplification and manipulation. It is understood that the input and output terminals of this device may be reversed, without changing its operating characteristics.
Typically, MEMS resonator filter devices 10 are fabricated by standard integrated circuit masking/ deposition/etching processes. Details regarding the manufacture and structure of MEMS band-pass filters may be found in the following references: 1) C. T. -C. Nguyen, L. P. B. Katehi and G. M. Rebeiz "Micromachined Devices for Wireless Communications", Proc. IEEE, 86, 1756-1768; 2) J. M. Bustillo, R. T. Howe and R. S. Muller "Surface Micromachining for Microelectromechanical Systems", Proc. IEEE, 86, 1552-1574 (1998); 3) C. T. -C. Nguyen,"High-Q Micromechanical Oscillators and Filters for Communications", IEEE Intl. Symp. Circ. Sys., 2825-2828 (1997); 4) G. T. A. Kovacs, N. I. Maluf and K. E. Petersen, "Bulk Micromachining of Silicon", Proc. IEEE 86, 1536-1551 (1998); 5) K. M. Lakin, G. R. Kline and K. T. McCarron, "Development of Miniature Filters for Wireless Applications", IEEE Trans. Microwave Theory and Tech., 43, 2933-2939 (1995); and, 6) A. R. Brown, "Micromachined Micropackaged Filter Banks", IEEE Microwave and Guided Wave Lett.,8, 158-160 (1998).
The reference 7) N. Cleland and M. L. Roukes, "Fabrication of High Frequency Nanometer Scale Mechanical Resonators from Bulk Si Crystals", Appl. Phys. Lett, 69, 2653-2655 (1996) describes the advantages of using single crystal resonators as band-pass filters. The references 8) C. T. -C. Nguyen, "Frequency-Selective MEMS for Miniaturized Communication Devices", 1998 IEEE Aerospace Conf. Proc., 1, 445-460 (1998) and 9) R. A. Syms, "Electrothermal Frequency Tuning of Folded and Coupled Vibrating Micromechanical Resonators, J. MicroElectroMechanical Sys., 7, 164-171 (1998) both discuss the effects of heat on the stability of micro-mechanical band-pass filters. Of particular relevance as noted in these references is the acknowledgment that the existing processes for making MEMS bandpass filters have serious drawbacks. For instance, as most resonators are made of polycrystalline or amorphous materials to simplify fabrication, there is exhibited an increase in mechanical energy dissipation which softens the natural frequency of oscillation, as noted in above-mentioned references 1)-3). Etching polycrystalline materials does not allow for device features smaller than the polycrystalline grain size, which creates rough surfaces and prevents precise mechanical characteristics. For example, references entitled "Micromachined Devices for Wireless Communications", "Surface Micromachining for Microelectromechanical Systems", both detail the problems when polycrystalline material is used in MEMS resonators. Additionally, in reference 7) there is described the construction of resonators made of single-crystal silicon including a description of an attempt to use complex dry-etch techniques to obtain single-crystal resonators. The reference reports such resonator structures having scalloped edges, which reduces the precision of the final mechanical performance to that of polycrystalline structures. That is, their etch-process produced surface roughness that was similar to that of polycrystalline materials.
Other attempts to use single-crystal silicon have been reviewed in reference 4). However, these attempts were made to eliminate the poor device performance when polycrystalline materials were used for construction. Most used anisotropic etches to undercut single-crystal silicon surfaces and construct resonators (and other structures). In all cases, the structures were quite large, in part to minimize the effects of surface roughness and non-parallel surfaces on the device performance. Since the devices were very large, they were useful only for low-frequency applications (below 100 MHz), which is of limited usefulness as a communication frequency filter in the commercial band of 300-6000 MHz.
A further limitation of all MEMS band-pass structures is that they are formed above the silicon surface (see references 1-9), making the structures incompatible with standard integrated circuit fabrication, since it prevents "planarization". After the devices of an integrated circuit have been fabricated, the wafer enters its final processing which is called "metallization" and "planarization". Before this step, all the devices on the wafer are isolated, and for integration they must be connected together with metal wires. In modern devices, the wiring is done as a series of layers, each containing wiring in certain directions (i.e., metallization). After each layer is deposited, the wafer surface is smoothed, i.e., is planarized so that subsequent layers of wiring may be deposited on a smooth surface. Planarization is typically done by chemical-mechanical polishing (CMP processing) or by melting a thin layer of glass over the surface. If there is a micro-mechanical device protruding up above the surface, it would be destroyed by either of the above planarization processes.
Finally, most MEMS band-pass filter structures have been "open" structures, i.e. on top of the substrate and operating in air. References 1)-3) report that operating resonators in air adds significant friction to the system, reducing the device efficiency.
Prior patented devices such as described in U.S. Pat. No. 3,634,787 (1972), U.S. Pat. No. 3,983,477 (1976), and U.S. Pat. No. 4,232,265 A (1980) describe similar mechanical resonatored structures, but which are incompatible with integrated circuit processing.
For instance, U.S. Pat. No. 3,634,787 describes an electro-mechanical resonator band-pass filter device having a mechanical component consisting of a support being a unitary body of semiconductor material and having a piezoelectric field effect transducer therein. Thus, its electrical operation relies upon the piezoelectrical effect. U.S. Pat. No. 3,983,477 describes a ferromagnetic element tuned oscillator located close to a high-voltage current carrying conductor, however, as such, its electrical operation relies on the ferromagnetic effect. U.S. Pat. No. 4,232,265 A describes a device for converting the intensity of a magnetic or an electromagnetic field into an electric signal wherein movable elements are made as ferromagnetic plates. Likewise, its electrical operation relies upon the ferromagnetic effect. U.S. Pat. No. 5,594,331 describes a self-excitation circuitry connected to a resonator to process induced variable frequency voltage signals in a resonant pass band and is of exemplary use as a power line sensor. Likewise, U.S. Pat. No. 5,696,491 describes a microelectromechanical resonating resonator which responds to physical phenomenon by generating an induced variable frequency voltage signal corresponding to the physical phenomenon and thus, does not lend itself to manufacture by current integrated circuit fabrication technology.
It would thus be highly desirable to construct an IC MEMS filter device in a manner consistent with current integrated circuit fabrication techniques that avoids completely or reduces significantly all of the above-described limitations.
Furthermore, it would be highly desirable to construct an IC MEMS filter device in a manner consistent with current integrated circuit fabrication techniques including features for removing problems of environmental contamination and also the deleterious effects of air on the vibrating resonator.